The present invention relates to a focus servo control device for an optical disk apparatus.
When information is recorded on or reproduced from an optical disk in a conventional optical disk apparatus, the optical disk is rotated while being irradiated with a laser beam. The laser beam must always be focused on the recording surface of the optical disk. From a microminiaturization point of view, the surface of the optical disk can be seen not to be a completely flat surface. Indeed, undulations cause the recording surface to move vertically while the disk is being rotated, making accurate focusing impossible with a fixed focusing device. For this reason, a focus servo control is required. Since a lens with a focal depth of .+-.2 .mu.m is used as an objective lens in the focus servo control, the laser beam must be focused on the recording surface of the optical disk to a precision of .+-.2 .mu.m.
The servo control comprises of three functional sections (a) a focusing error detecting section; (b) a servo amplifier section; and (c) a focusing section. A focus error is detected by section (a). A detection signal is amplified and its frequency characteristics controlled by section (b). A focusing mechanism is driven by section (c). The focusing mechanism has a member that moves upon supply of an excitation current, in the same manner as the voice coil of a speaker. The objective lens is moved vertically by the member, thereby performing focusing.
With a conventional focus servo control system, a focus servo circuit is operated when a focusing mechanism-an objective lens-is moved to a position within a focus servo control range. However, offset errors produced by an optical system and an electrical circuit are effectively superposed on the focus error signal. Therefore, a proper focus state cannot be maintained because of the offset errors.